Gasification of sulphite thick liquor

ABSTRACT

Method for recovering chemicals and energy from sulphite thick liquor containing organic and inorganic compounds obtained when producing pulp by chemical delignification of fibrous raw material using a sulphite pulping process, the method including processing of the organic and inorganic compounds at a global temperature above 800° C. whereby producing partly at least one phase of a liquid material and partly at least one phase of a gaseous material. The processing is carried out by gasification of the sulphite thick liquor in a gasification reactor at sub-stoichiometric conditions and in the presence of an oxidizing medium. The reactor has an opening in its bottom in the form of a chute which opens directly into a quench compartment.

This application is a Continuation-In-Part of international applicationNo. PCT/SE2011/050350, filed 29 Mar. 2011. This application also claimsforeign priority to Swedish patent application No. SE 1050299-5, filed30 Mar. 2010. The complete contents of these applications isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for recovering chemicals andenergy from sulphite thick liquor, said sulphite thick liquor beingobtained when producing paper pulp by chemical delignification offibrous raw material using a sulphite pulping process, said sulphitethick liquor comprising organic and inorganic compounds; the methodcomprising processing of said organic and inorganic compounds at atemperature above 800° C. thereby producing partly at least one phase ofa liquid material and partly at least one phase of a gaseous material.

BACKGROUND INFORMATION

The sulphite pulping process is a chemical pulping process of wood chipswhen producing pulp. Sulphite pulping currently accounts for less than10% of the world's pulp production, although it has historically been adominating process for pulping before important developments in theKraft (also termed sulphate) pulping process made this process morepopular. Different sulphite pulping processes exist, e.g. acidic,neutral and alkaline sulphite processes.

In sulphite pulping processes using mixtures of sulphur dioxide,sulphurous acid and/or its alkali salts, the lignins in the wood chipsare made water-soluble through the formation of sulphonatefunctionalities and cleavage of bonds in the lignin structures. Typicalcounter-ions to the sulphurous acid and/or its alkali salts include Na⁺,NH⁴⁺, Mg²⁺, K⁺ and Ca²⁺.

A main difference between the different sulphite pulping processes arethe pH of the cooking liquor which means that delignification is carriedout at low, neutral or high pH in the digester Alkaline sulphite pulpingmay also be performed in the presence of sulphide, so calledsulphide-sulphite pulping.

The sulphite pulping process may have certain advantages compared toKraft pulping, such as higher yield, brighter and more easily bleachedpulps and relatively easily refined pulps. The sulphite process mayproduce specialty cellulose for production of cellulose derivatives inaddition to pulp for papermaking. Certain disadvantages compared toKraft pulping also exist, such as weaker pulp and difficulties inpulping certain species of wood and more complicated chemical recovery.

The chemical recovery process of the pulping chemicals is dependent onthe alkali counter-ion used but is generally more complex than the Kraftprocess recovery of pulping chemicals. The initial steps in the sulphitechemical recovery process are separation of spent cooking liquors fromthe pulp/cellulose and subsequent concentration of the spent liquor byevaporation of water, which gives a liquor denoted sulphite thick liquorin this text. The sulphite thick liquor may thereafter, for most counterions, be burned in recovery boilers for energy recovery and the pulpingchemicals are recovered to varying extent.

The recovery boilers used for recovering chemicals and energy fromsodium-based sulphite thick liquors are very similar to those used forrecovery of black liquor from the Kraft pulping, however burningsulphite thick liquor in such boilers is associated with a number ofdifficulties as compared to burning Kraft black liquor, which is furtherdiscussed below.

The flue gases produced when burning the sulphite thick liquor are morecorrosive, which limits the efficiency of liquor energy recovery andcauses elevated maintenance costs.

The losses of pulping chemicals, both sodium and sulphur, mostly as flyash, are significantly higher when burning sodium-based sulphite thickliquor as compared to burning Kraft liquor, which can lead to increasedchemical make-up costs in the mill.

Burning sodium-based sulphite thick liquor to recover the cookingchemicals and energy, is a high temperature process where the salt meltcollected in the bottom of the boiler needs to be kept at hightemperatures (around 1000° C.) due to the high melting point of thesulphide/carbonate mixture formed.

Reduction of the sulphur components in recovery of spent liquors fromthe Kraft process can normally reach 95%. When burning sodium-basedsulphite thick liquor, the reduction efficiency of sulphur species inthe sulphite liquor is relatively low. Typically 80-85% of the recoveredsulphur is reduced to sulphide that can be converted to active cookingchemicals in subsequent cooking liquor preparation process. Thenon-reduced sulphur gives disadvantages in the form of dead load in theliquor cycle and a tendency to cause fouling in the process equipment ofthe liquor cycle.

The non-reduced sulphur is, at least partly, present as polysulphide inthe salt melt, which is oxidized to tiosulphate in the green liquorformed by the dissolved salts coming from the recovery boiler.Tiosulphate decreases pulping efficiency if present in the cookingliquor. To avoid such effects, wet oxidation is used to converttiosulphate to sulphate. Hence, a large amount of non-active sulphur ispresent in the liquor cycle, causing a lower efficiency and potentialproblems with scaling. In addition, thiosulphate is known to causecorrosion problems in process equipment.

Sulphite thick liquors are known to have a lower reactivity in recoveryboilers compared to spent Kraft cooking liquors, which leads to lowercapacity when recovery boilers are operated on sulphite thick liquor. Akey reason why the sulphite liquor behave differently than the Kraftliquor is normally meant to be caused by less swelling behavior of thesulphite liquor droplets during heat up before combustion, which leadsto higher resistance to mass and energy transfer.

Hence the complex and relatively inefficient chemical and energyrecovery from spent sulphite liquors is one reason why the Kraft processhas become the dominating pulping process.

Furthermore, persons skilled in the art may have a prejudice againstrecovery by gasification of sulphite liquors based on earlierexperiences. Tests have for instance been carried out already around1960 by the Swedish pulp and paper company Billerud and two separatepathways were further explored. A non-slagging (low temperature)gasification process was developed and built in a few facilities (the“SCA-Billerud process”). The slagging (high temperature) pathway wastested in a second facility at the Billerud Mill. Tests were ended afterone year due to a combination of factors. The process did not reach thelow smelt sulphide content requirements set by the remaining recoveryprocesses available at that time. Further, problems with build-up ofsmelt layers on the reactor walls were present and the wear on thereactor lining was very severe with the ceramic materials available atthat time. Also the “SCA-Billerud process” was subsequently abandoneddue to poor performance.

In document U.S. Pat. No. 2,285,876, a process for recovery of wastesulphite liquors is disclosed. Said liquor is sprayed into a so calledTomlinson recovery furnace chamber and burnt at a furnace temperaturebelow the fusion temperature of the non-combustible constituents of saidliquor.

Document DE 1,517,216 describes a process for pyrolysis of cellulosespent liquors, especially of sodium based sulphite thick liquors.Thickend spent liquor is divided into very fine particles where themajor part of the particles should not exceed 200 μm, said particlesbeing sprayed into a hot oxygen containing gas stream and beingpyrolized. The document teaches that the pyrolysis temperature shouldnot exceed 800° C. in order to avoid sulphides in the solid residue thatis used to make green liquor and, consequently, in the cooking liquor.Pyrolysis at as low temperatures as below 800° C. will however lead tounconverted char in the solid residue from the gasification process andnecessitates a second gasification step that is performed in a fluidizedbed. The hot gas into which the liquor is added comes from combustion offuels, e.g. oil.

Document, U.S. Pat. No. 3,317,292, describes a method of treating wastesubstances, such as sulphite waste liquor, black liquor etc, to derivehydrogen and other gases therefrom as well as a hydrogen-containingproduct. The method comprises precipitating lignin-derived components,reacting the precipitate with steam at several hundred degrees in areaction atmosphere substantially void of free oxygen to favourproduction of carbon monoxide and hydrogen gases.

Another document SE 526435 discloses a method for recovery of chemicalsfrom alkaline sulphite pulping processes. Said method comprises agasification step and the document teaches that said gasification shallbe carried out at a temperature of preferably 700-750° C. in order tokeep the temperature below the melting point of the salts in the solidphase.

Still another document, CA 619,686, discloses a method for pyrolysis ofwaste liquors from pulp manufacturing, preferably on sodium base, byusing a fluidized bed.

In document WO 86/07396 a process for gasification of black liquor isdisclosed. The document teaches that introduction of oxygen oroxygen-containing gas must take place at some minimum distance from thepoint of introducing the black liquor into the reactor so as to avoidoxidation of sulphides contained in the pyrolysed droplets. The documentWO 00/75421 discloses a reactor for gasification of spent liquor wherethe reactor part has substantially the same, open cross sectional areaas at a higher level, above.

Taking the above into consideration there is a need to improve thechemical recovery process for sulphite pulping and to increase theefficiency with regard to energy and/or chemical recovery.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at leastminimize at least one of the drawbacks and disadvantages of the abovedescribed chemical recovery processes for sulphite pulping. This can beobtained by a method according to claim 1.

Thanks to the invention a more efficient chemical recovery is obtained.Cold gas efficiency obtainable in a commercial scale gasifier isestimated to be 65-75%, leading to high yields of motor fuels producedfrom the synthesis gas, if this usage of the synthesis gas is selected.Explanations to the high energy efficiency of the process may includethe burner design of the gasification reactor that allows operation withhigh carbon conversion at a relatively low global reactor temperatureand also allows gasification without any additional atomizing medium.

Green liquor sulphidity is significantly lower than for a recoveryboiler due to the fact that most of the sulphur may be contained in theraw synthesis gas as hydrogen sulphide. This split of the thick liquorsulphur content is caused by chemical reactions between components ofthe gas and smelt phases in the reactor, which determine the proportionsof liquid sodium sulphide and gaseous hydrogen sulphide. The sulphur inthe gas may be returned to cooking liquor preparation in a concentratedgas stream from an acid gas removal unit treating gas from the gasifier,which permits a less complex cooking liquor preparation process. Theload on the part of the cooking liquor preparation that converts greenliquor sulphide to sulphur dioxide and sulphurous acid is decreased dueto the lower sulphur content in the green liquor.

According to one aspect of the invention, the amount of unburnt charcoalin said green liquor is lower than 5%, preferably lower than 1% and morepreferred lower than 0.2%, of the carbon in the sulfite thick liquor,i.e. carbon conversion may be very high, resulting in a good qualitygreen liquor. The high carbon conversion is obtained because of the highflame temperature in the reactor caused by rapid chemical reactions ofoxygen or oxygen-containing gas with combustible components and anadvantageous recirculating flow pattern in the reactor chamber forced bythe limited size of the reactor chamber bottom outlet.

According to another aspect of the invention, the sulphur found in thegreen liquor may to an extent of at least 90%, preferably at least 95%and more preferred at least 98%, be in reduced form, i.e. as sulphide.This means that the green liquor produced may have close to 100% sulphurreduction efficiency. High sulphur reduction efficiency increases theefficiency of the pulping process, since it decreases the total amountof sulphur that needs to be circulated by decreasing the so-calleddead-load (i.e. inactive sulphur species such as sulphate). The highsulphur reduction efficiency is obtained because of an advantageousrecirculating flow pattern in the reactor chamber forced by the limitedsize of the reactor chamber bottom outlet.

All these advantages taken together leading to a more efficient and costeffective chemical recovery process with regard to cooking chemicals aswell as energy. Said chemical recovery process may no longer be adrawback for sulphite pulping processes compared to Kraft (sulphate)pulping processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a flow scheme of a typical chemical recovery cycle foracidic sodium sulphite pulping,

FIG. 2 shows a flow scheme of a modified and more efficient cycleincluding gasification of sulphite thick liquor according to theinvention, and

FIG. 3 shows a general process scheme on a gasification plant of theentrained-flow, high temperature reactor type

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description, and the examples contained therein,are provided for the purpose of describing and illustrating certainembodiments of the invention only and are not intended to limit thescope of the invention in any way.

In FIG. 1 is a flow scheme of a typical chemical recovery cycle forsodium-based sulphite pulping shown. Since this is common knowledge fora person skilled in the art, said chemical recovery cycle will be onlybriefly described here. Wood chips A are pumped into a digester wherethe delignification/pulping process B takes place in an appropriatesulphite cooking liquor at elevated temperature thereby releasingcellulose fibres, pulp, C. The pulp is separated from the spent cookingliquor, also termed thin sulphite thick liquor D, which spent liquor isa mixture of spent cooking chemicals and wood residues (e.g. lignins).The raw pulp C is then ready but may often be further treated inbleaching units and may thereafter be exported either as wet or drypulp.

The spent cooking liquor is cooled and stripped of free sulphur dioxidein an evaporator. The pre-evaporated and cooled liquor is fermented toreduce the sugar content of the liquor (not shown). After fermentationthe liquor is stripped to recover the ethanol formed and evaporated inone or several evaporators E to a sulphite thick liquor F with about60-70% dry solids. The liquor is then combusted in one or severalTomlinson-type recovery boilers G to form smelt, flue gases H comprisingash and heat for steam generation.

The smelt substantially comprises sodium carbonate and sodium sulfide.Said smelt is dissolved in recirculated water. The solution, so calledgreen liquor I, is clarified or filtered to remove insoluble inorganicsubstance and any char from incomplete combustion in the recoveryboilers. The green liquor is then stripped from its content of sulphideby contacting it in countercurrent with carbon dioxide and sulphurdioxide recovered from the Claus plant (this and the following are notshown explicitly in FIG. 1 but is a part of the cooking liquorpreparation). The resulting gas, a mixture of hydrogen sulfide andcarbon dioxide is led to the Claus plant where it is contacted withsulphur dioxide to form elemental sulphur. The elemental sulphur iscombusted in a sulphur furnace to form sulphur dioxide which through anadsorption-desorption system is concentrated and then led to the Clausplant. The flue gas from the recovery boiler which is rich in sulphurdioxide is contacted in a flue gas scrubber with the sodium sulfitesolution from the above mentioned sulfide stripper column to form amixture of sulfite and bisulfate. Subsequently this mixture is contactedwith sulphur dioxide from the sulphur dioxide absorber to form abisulphate solution. A fresh cooking liquor L comprising sodiumsulphite, bisulphate and/or bisulphate with free sulphur dioxide maythen be prepared in the cooking liquor preparation step J. Make-upsodium and sulphur K may be added to the preparation J. Said freshcooking liquor L is now ready to be conveyed to the pulping process B.

In FIG. 2, a flow scheme of the preferred embodiment according to theinvention is shown where the recovery boiler has been replaced by one orseveral gasifier(s) for gasification M of the spent liquor therebyforming smelt and raw synthesis gas N. Said smelt is dissolved inrecirculated water or in weak wash liquor, thereby forming green liquorI, in the same way as in the conventional chemical recovery cycle shownin FIG. 1. Said raw synthesis gas N passes through a gas cleaningplant/process O, which may be a so called acid gas removal plant (AGR),resulting in cleaned synthesis gas P and a hydrogen sulphide rich gas Q,said hydrogen sulphide rich gas being fed to the cooking liquorpreparation J, where it is converted to sulphur dioxide and/orsulphite/bisulphate to be used as cooking chemical. Another streamcomprising mainly carbon dioxide R and essentially free from sulphurcompounds may also be produced in the plant/process O.

Adding the step of gasification of sulphite thick liquor to the chemicalrecovery process for sulphite pulping may potentially give a much moreefficient recovery process, both with regard to cooking chemicals andenergy. It is understood that said gasification step may either replacethe recovery boiler or be included in the already existing chemicalrecovery process for sulphite pulping as a booster. In many mills thechemical recovery process is a bottle neck thereby limiting the pulpproduction which cannot be further increased.

Since the gasification process and gasification plants themselves arewell-documented, they will be described only briefly here.

FIG. 3 shows a general process scheme of a gasification plant of theentrained-flow type for gasification at slagging conditions (hightemperature) in accordance with the invention. Said plant being a partof the chemical recovery cycle for sulphite pulping shown in FIG. 2. Theplant corresponds to the parts denoted I, M, N, O, P, R and Q in FIG. 2.

In FIG. 3 the gasification plant is shown in more detail and is to bedescribed below. The description is to be seen as a general descriptionof a gasification plant and shall be interpreted as illustrative and notin a limiting sense. It is to be understood that numerous changes andmodifications may be made to the below described plant, withoutdeparting from the scope of the invention, as defined in the appendingclaims.

Detail number 1 in FIG. 3 denotes a pressure vessel which comprises aceramically lined gasification reactor 2. The reactor is provided withan inlet 3 for sulphite thick liquor and an inlet 4 for oxygen oroxygen-containing gas, which are both connected to a burner (not shown).There may also be an inlet 40 for additional atomizing medium, e.g.atomizing steam. Additional atomizing medium may also be mixed with theoxygen or oxygen-containing gas before said gas is supplied to the inlet4. Additional atomization support may lead to smaller thick liquordroplets and a more efficient process.

The opening in the bottom of the reactor chamber is limited in size togive a recirculating flow pattern in the reactor, which is required togive high carbon conversion and sulphur reduction efficiency.Preferably, said opening has a horizontal cross sectional diameter whichis less than 40%, and more preferred being 5-35%, of the diameter of thelargest cross sectional area in a horizontal plane of the reactor 2. Theopening is in the form of a chute 5, which opens directly into a quenchcompartment 38 above the surface 35 of the liquid in a green liquorliquid chamber 6 which is situated below. Said quench compartment 38 isan integrated portion of the reactor 2. One purpose of the quenchcompartment 38 is to cool the gas leaving the reactor to a temperatureat which gas phase chemical reactions does not take place at asignificant rate. Cooling of the gas also leads to less stringentmaterial requirements in the construction of the quench compartment 38,which is economically and technically advantageous.

A number of spray nozzles 7 for cooling liquid open out into the chute 5and the quench compartment 38. Green liquor which is produced istransported from the chamber 6 through a conduit 8, via a pump 9 and aheat exchanger 10, to subsequent process stages for generating cookingliquor, or to another process stage in which green liquor is employed. Apartial flow of the green liquor transported in conduit 8 may bereturned to the green liquor liquid chamber 6 through a conduit 81 via apump 91. Cooling liquid that is not evaporated may be collected in avolume 36 to be reused.

Raw synthesis gas from the first vessel is conveyed through a conduit 11to a second pressure vessel 12 for gas treatment and sensible/latentheat energy recovery.

This conduit 11 opens out in the pressure vessel 12 above the surface ofa liquid in a washing chamber 13 at the bottom of the vessel.

The liquid in the washing liquid chamber of the second vessel may beconveyed, through a conduit 14 via a pump 15, to the first vessel inorder to serve as diluting liquid or as a cooling liquid which isprovided via the spray nozzles 7.

The pressure vessel 12 may comprise an indirect condenser of thecountercurrent falling-film condenser type 16 located above the chamber13. Other types of condensers may be used without departing from thescope of the invention and since methods for gas washing and gas coolingare well known techniques it will not be described in detail here.

An outlet conduit 17 for cooled raw synthesis gas is located at the topof the second pressure vessel 12. The outlet conduit 17 transports thecooled combustion gas to an inlet 31 of a plant 30 for further removalof sulphurous components and most of the CO₂. The plant 30 comprises anygas separation technology for acid gas removal (AGR). A conduit 32 ofthe plant 30 may transport the purified and cooled synthesis gas, nowcalled cleaned synthesis gas, to any field of use of the synthesis gas,e.g. chemical production, fuel production, electricity generation and/orsteam/heat generation. Hydrogen sulphide and carbon dioxide (denoted Qand R, respectively, in FIG. 2) removed from the cool raw synthesis gasmay leave the plant 30 via conduits 33 and 34, respectively. Saidremoved hydrogen sulphide may then be conveyed to the cooking liquorpreparation while said carbon dioxide may be conveyed back to the milland be used where appropriate, e.g. stripping hydrogen sulphide from thegreen liquor in the recovery process.

The gasification-based recovery process is now to be described. Thegasifying reactor is fed with concentrated sulphite thick liquor, saidsulphite thick liquor comprising organic and inorganic compounds,together with oxygen or an oxygen containing gas that may be pre-heatedto 50-400° C. The thick liquor feed pressure is typically 0-20 bar overreactor pressure to account for the pressure in the gasification processplus the burner pressure drop. Suitable feed pressure for thick liquormay be accomplished by a pump as indicated in the uppermost part of thereactor 2 in FIG. 3.

The liquor is processed by gasification in the presence of an oxidizingmedium, e.g. oxygen or air, whereby heat is released by the chemicalreactions taking place to give a global temperature above 800° C.,preferably above 900° C., more preferred above 950° C. but below 1500°C., preferably below 1300° C., and at an absolute pressure of about 1.5to about 150 bar, preferably about 10 to about 80 bar, and mostpreferably from about 24 to about 40 bar in the reaction zone (a socalled high pressure gasification).

Normally feedstock atomization is achieved by the flow of the oxygen oroxygen-containing gas but an atomizing support medium may be used.Atomization is preferably accomplished by accelerating the flow ofoxygen or oxygen-containing gas in the burner and utilizing the kineticenergy thus generated. The oxygen or oxygen-containing gas feed pressureis typically 1-50 bar over reactor pressure to account for the pressurein the gasification process plus the burner pressure drop. The sulphitethick liquor forms droplets when meeting said oxidizing medium, i.e. theoxygen or oxygen containing gas, and it is preferred that said dropletshave an average droplet size below 300 μm. A small droplet size isbeneficial, since it leads to more rapid chemical reactions between thethick liquor and oxygen or other gas components, which is advantageousfor e.g. carbon conversion. The inlet for oxygen or oxygen-containinggas is preferably located to efficiently use the kinetic energy of thegas flow for atomization of the sulphite thick liquor. This may beachieved by positioning the inlets for oxygen or oxygen-containing gasand sulphite thick liquor close to each other so that the gas stream isnot slowed down before it interacts with the sulphite thick liquor, i.e.by introducing said oxidizing medium to an inlet of the reactor 2 invicinity of an inlet of the reactor 2 for introduction of sulphite thickliquor F so that the oxidizing medium can be used for atomization of thesulphite thick liquor.

Said gasification takes place at reducing conditions, i.e.sub-stoichiometric oxygen conditions, thereby producing a mixture ofpartly at least one phase of a liquid material and partly at least onephase of a gaseous material. It may be beneficial that the reactorbottom outlet is designed to give a recirculating flow pattern in thereactor in order to achieve the desired process performance. The bottomof the reactor and the opening to the chute 5 is preferably arranged sothat liquid material that may be flowing on the reactor walls can leavethe reactor without obstruction and is not accumulated in the bottompart of the reactor.

The phase of gaseous material comprising raw synthesis gas, e.g. carbonmonoxide, hydrogen, carbon dioxide, methane, hydrogen sulphide, andaqueous steam, and the phase of liquid material comprising inorganicsmelt and ash, e.g. sodium sulphide, carbonate and hydroxide, are cooledin the quench cooler zone by spraying cooling liquor through a number ofnozzles in order to achieve maximum contact with the gas/smelt mixture.The cooling liquid principally consists of water, some of which waterwill be evaporated when it makes contact with the hot gas and the smeltat the reactor temperature. The gas temperature drops to approx.100-230° C. in the quench cooler zone. The smelt drops may be dissolvedin the remaining part of the cooling liquid and falls into the greenliquor liquid chamber (the so called quench bath) where it dissolves toform green liquor. Alternatively, the smelt drops fall down directlyinto the liquid chamber and only then dissolve in the green liquor whichis already present in this location. The smelt drops are then possiblycooled by the evaporation of water in the green liquor bath.

The green liquor comes out from the bottom of the quench cooler of thefirst pressure vessel through a conduit and may be pumped through a heatexchanger, in which heat energy is recovered from the green liquor bycooling the latter. Alternatively, green liquor heat energy may berecovered by other means. A screen may be used ahead of the pump tocatch small particles.

It is beneficial that the amount of unburnt charcoal in the smelt and insaid green liquor is lower than 5%, preferably lower than 1% and morepreferred lower than 0.2%, of the carbon in the sulfite thick liquor.i.e. that the carbon conversion in the reactor is at least 95%,preferably at least 99% and more preferred at least 99.8%. Carbonconversion is determined by a multitude of factors of which the mostimportant are atomization (liquor droplet size), local flametemperature, reactor flow pattern and global reactor temperature. Thedesign of the burner is important for high carbon conversion since itinfluences atomization and local flame temperature.

The global reactor temperature is defined as the temperature of the gasand smelt leaving the reactor at the bottom outlet, which is determinedby the energy balance of the reactor. The local flame temperature isdefined as the temperature in a flame zone of the reactor, i.e. close tothe burner where rapid exothermal chemical reactions take place leadingto local temperatures much higher than the global reactor temperature.The flame temperature is determined by mixing and reaction rates in theflame zone.

The green liquor sulphide is recovered in the same manner as thesulphide in the green liquor from a recovery boiler connected to asulphite pulping process, i.e. by stripping the green liquor from itscontent of sulphide by contacting it in countercurrent with carbondioxide and sulphur dioxide, preferably in an absorption/desorptiontower, and then further to the pulping chemicals sulphur dioxide and/orsulphite, but the lower sulphidity of the gasification green liquor (dueto the sulphur content in the raw synthesis gas) leads to lower capacityrequirements in the equipment used for this purpose. In addition, a highsulphur reduction efficiency decreases the total amount of sulphur thatneeds to be circulated by decreasing the so-called dead-load (i.e.inactive sulphur species such as sulphate). It is beneficial that thesulphur found in the green liquor is to an extent of at least 90%,preferably at least 95% and more preferred at least 98%, in reducedform, i.e. as sulphide, i.e. that the sulphur reduction efficiency is atleast 90%, preferably at least 95% and more preferred 98%.

It must be noted that the problem of reaching high sulphur reductionefficiency is different and more difficult in the case of sulphite thickliquor gasification compared to black liquor gasification. This iscaused by the oxidation state of sulphur in the feedstock liquor. Forblack liquor, a large fraction of the sulphur is already present assulphide. For sulphite thick liquor, on the other hand, practically allsulphur is in an oxidized state since the pulping chemical is sulphiteor bisulphite. This means that for black liquor gasification, theproblem partly consist of not oxidizing reduced sulphur, while forsulphite thick liquor gasification all sulphur must be reduced in orderto reach high sulphur reduction efficiency.

A minor part of the green liquor may be employed for wetting the insideof the chute by means of being returned to the chute and being permittedto form a thin film on the inside of the chute. The formation of thisfilm may be accomplished by different means. The purpose of the film maybe both corrosion prevention and process performance.

The raw synthesis gas, leaving the primary quench dissolver of thereaction vessel, now essentially free of smelt drops, is further cooledto saturation in the second vessel 12 a gas cooler for particulateremoval and gas cooling. Water vapour in the raw synthesis gas iscondensed, and the heat released may be used to generate steam and/orhot water.

Hydrogen sulphide and carbon dioxide are removed from the cool rawsynthesis gas in a so called acid gas removal plant—AGR. Several knowncommercial gas cleaning systems comprising units for absorption of acidgas and recovery of sulphur may be used. Said removed hydrogen sulphidemay then be conveyed to the cooking liquor preparation.

It is beneficial that the hydrogen sulfide rich stream removed from thecool raw synthesis gas in the AGR comprises at least 25% hydrogensulfide, preferably at least 35% hydrogen sulfide of the total streamcontent, since a high concentration of hydrogen sulphide facilitates thethereafter following steps of combustion and scrubbing.

Said carbon dioxide being removed from the cool raw synthesis gas in theAGR may be conveyed back to the mill and be used where appropriate, e.g.stripping hydrogen sulphide from the green liquor in the recoveryprocess.

The resulting synthesis gas is a nearly sulphur-free synthesis gascomprising carbon monoxide, hydrogen and only small amounts of carbondioxide, and may be used as feedstock for automotive fuels, chemicals orelectricity generation.

With a gasifier plant with an AGR several simplifications of the systemfor recovery of pulping chemicals may be made. The load on the part ofthe cooking liquor preparation that converts sulphide to sulphur dioxideand sulphurous acid is decreased due to the lower sulphur content in thegreen liquor. A Claus plant would not be needed to recover the sulphurin hydrogen sulphide form, since the hydrogen sulfide rich stream fromthe AGR may be combusted directly with air/oxygen to give sulphurdioxide of sufficiently high concentration that can be absorbed from thegas in a scrubber. The AGR replaces the recovery boiler flue gasscrubber function. Part of the carbon dioxide stream from the AGR(R) mayalso be used for carbonation in the cooking liquor preparation ifdesired.

Experimental

Pilot test of gasification of sulphite thick liquor In the presentinvention experimental tests of gasification of sulphite thick liquorfrom sodium based sulphite cellulose production were carried out but itis understood that other sulphite liquors, e.g. ammonium or potassiumbased sulphite liquors, may as well be used without departing from thescope of the invention.

In the test, the sulphite thick liquor originated from a processutilizing a sodium bisulphite-sulphite cooking liquor.

Sulphite thick liquor was transported in an insulated truck from thepulp mill to the pilot plant. 62% dry solids (DS) content was used,since long term stability is not verified at a concentration of 70% DS.Liquor was filtered through a 2 mm screen and kept in an agitatedinsulated tank from which liquor for gasification was taken.

The primary parameters studied are liquor load and reactor temperature.The test procedure used is based on stepwise increase in liquor loadfrom a relatively low starting point. The reactor pressure was increasedsimultaneously to keep reactor residence time comparable. Temperaturevariations, induced by varying O₂/liquor ratio, were used to study theinfluence of this factor.

The pilot gasifier has a burner for introduction of both liquorfeedstock and oxygen. Atomization was achieved by oxygen with somenitrogen mixed in. Varying amounts of nitrogen were used to study theinfluence of atomization on the process.

Prior to atomization, the liquor to be gasified is pre-heated todecrease viscosity and increase reactor energy efficiency. Fouling ofsurfaces in a heat exchanger used for this purpose was evident duringthe experiment which limited the obtainable load. Thus, testing ofmaximum reactor capacity could not be achieved in this experiment.Problems with surface fouling when indirect heating is used are wellknown from sulphite thick liquor handling at sodium sulphite mills andnot specific for gasification.

Operating points according to Table I were tested. The total duration ofthe test was 27 h. Start-up, operating point changes and shutdownconstituted approximately 5 h. Operating parameters not shown in Table Iwas not varied systematically. Green liquor circulation in the dissolversection, was significantly higher than normal.

TABLE I Operating points; some representative parameters showed. LoadLoad O₂/liq. Reactor Methane Liquor temp. Duration (wet) (dry) Pres.ratio^(A) temp.^(B) in cold gas after pre-heat h. kg/h t DS/d bar(g)kg/kg ° C. mol % ° C. 1 3 388 5.7 15 0.397 1010-1070 0.15%  128-130 2 2559 8.3 20 0.370 1010-1070 0.2% 121-124 3 3 559 8.3 20 0.359  980-10100.6% 121-122 4 2 559 8.3 23 0.359  980-1010 0.6% 123 5 2 631 9.3 230.374 1010-1070 0.2% 120 6 3 631 9.3 25 0.374 1010-1070 0.2% 119-121 7 6631 9.3 28 0.375 1010-1070 0.2% 119 8 0.5 631 9.3 28 0.350  980-10100.7% 119 ^(A)Based on wet liquor flow ^(B)Range measured by seventemperature sensors in the reactor

Liquor Analysis

A liquor sample for analysis was taken at the mill when the liquor wasshipped. The composition, shown in Table II, is representative fornormal mill operation except for the dry solids content, which asexplained above is lower than normal.

TABLE II Sulfite thick liquor analysis Liquor composition mass/mass HHVMJ/kg DS 17.5 C kg/kg DS 43.3% H kg/kg DS 4.2% S kg/kg DS 8.7% O kg/kgDS 33.9%^(B) Na kg/kg DS 8.8% K kg/kg DS 0.23% Cl kg/kg DS 0.01% N kg/kgDS 0.9% DS kg/kg wet 61.7% ^(B)By difference, not analyzed

Synthesis Gas

Gas composition was measured by on-line analyzers and sampling followedby gas chromatographic laboratory analysis. Only the results fromlaboratory analysis are discussed here since they are considered moreaccurate.

Cold synthesis gas was sampled at the end of each operating period(Table I). The results are shown in Table III. The high N₂ content isdue to specific pilot scale solutions, mainly instrument purge, and willnot be present in a full scale gasification process. The high CO₂/COratio is also a pilot scale effect due to high heat loss, as discussedfurther below.

TABLE III Cold gas composition as determined by gas chromatographicanalysis. Operating COS HHV^(B) LHV^(B) point CO₂ % H₂S % O₂/Ar % N₂ %CH₄ % CO % H₂ % ppm MJ/Nm³ MJ/Nm³ 1 26.6 1.77 0.0 31.5 0.11 18.7 20.0 665.43 4.99 2 26.6 2.07 0.0 23.0 0.28 21.9 24.3 62 6.58 6.05 3 26.3 2.040.1 23.1 0.56 21.5 24.6 66 6.68 6.13 4 26.2 2.06 0.0 23.1 0.56 21.4 24.768 6.70 6.14 5 27.1 2.06 0.0 21.2 0.23 22.4 24.9 50 6.72 6.18 6 27.32.09 0.1 20.1 0.22 22.9 25.3 48 6.83 6.28 7 27.6 2.09 0.0 20.1 0.18 23.025.5 48 6.83 6.28 8 26.2 2.13 0.0 20.5 0.67 22.7 26.3 58 7.10 6.51Uncert.^(A) 0.4 0.02 0.07 0.7 0.004 0.2 0.2 2.5 ^(A)Uncertainty given asstandard deviation estimated from duplicate analysis of four individualsamples ^(B)Calculated based on composition

Green Liquor

Green liquor samples were taken for chemical analysis (Table IV) and forvisual inspection and density measurement. Carbonate and hydrogencarbonate were determined by acid titration, sulphide by silver nitratetitration and total sulphur by wet oxidation followed by ionchromatography.

It should be noted that some difficulties are present when trying tocorrelate green liquor properties with gasifier operating conditions.The long residence time in the quench green liquor volume makesobtaining steady-state time consuming Only operating point 7 hassufficient duration to give a representative green liquor sample. Allother samples are considered to be influenced by several operatingpoints.

Carbon conversion was not measured explicitly but is considered to becomplete or almost complete in all operating points from the visualappearance of the green liquor. Unconverted carbon (char) is normallyclearly visible as non-settling black particulates when present in thegreen liquor even in small quantities.

The green liquor concentration is lower than what is normal for thepilot plant during Kraft liquor operation and compared to what used atthe cellulose mill today, which is mainly due to the difficultyassociated with controlling total titratable alkali (TTA) during theshort test duration and changing operating points. Operating at highergreen liquor concentrations is not believed to influence green liquorcomposition or quality significantly.

Green liquor sulfidity, measured as S/Na₂ ratio (mol/mol), isapproximately 0.5 on average. This corresponds to a HS⁻ concentrationthat is 25% of TTA. The CO₂-absorption is high due to the deliberatelyhigh green liquor circulation flow and gives HCO₃ ⁻ concentrations thatare about 30% of TTA. It is possible to control CO₂-absorption to alarge extent by quench design and operation, which can be used tooptimize the green liquor for cooking liquor preparation processes. Itshould be noted that CO₂-absorption is not a disadvantage as is the casefor Kraft green liquor since causticization is not used.

Reduction efficiency seem to not deviate from 100% within measurementaccuracy; cf. values in Table IV, which are based on analyses of totaland sulphide sulphur This a marked difference compared to the 80-85%reduction efficiency that are obtained in the mill recovery boilerspresently according to green liquor analyses.

TABLE IV Results from chemical analysis of green liquor samples.Operating TTA CO₃ ²⁻ HCO₃ ⁻ HS⁻ Tot-S S/Na₂ Red. point mol/l mol/l mol/lmol/l mol/l mol/mol efficiency 1 0.70 0.16 0.19 0.18 0.18 0.51 98% 21.31 0.33 0.32 0.33 0.33 0.51 101% 3 No analysis 4 2.21 0.53 0.64 0.500.49 0.46 103% 5 2.61 0.65 0.72 0.60 0.60 0.46 99% 6 2.40 0.57 0.69 0.570.55 0.47 104% 7 2.21 0.49 0.71 0.53 0.48 0.48 110% 8 No analysis

Analysis and Discussion Sulphur Split Ratio

The split of sulphur between gas and smelt may be a very importantparameter for mill integration and dimensioning of downstream gasprocessing equipment. The sulphur split ratio (defined here as thefraction of sulphur in the synthesis gas) may be calculated from sulphurcontent and flows in different combinations of streams. Alternatively,it may be possible to calculate the sulphur split from S/Na₂ ratios ingreen liquor exiting and thick liquor entering the system if it isassumed that all Na leaves the system in the green liquor stream.

The method based on S/Na₂ ratio is not dependent on flow measurements,which is an advantage. Calculations based on measured ratios indicatethat 69% of the sulphur ends up in the gas phase at 28 bar reactorpressure.

The sulphur split ratio for gasification of sulfite thick liquor issignificantly higher than the 30-40% obtained for Kraft black liquor.

Smelt Melting Point

Smelt composition determines the physical properties of the liquid phasein the reactor. There is a risk of solidification on “cold” surfaces inthe reactor exit if melting temperature is too high. An approximatesmelt composition can be determined from the green liquor analysis byassuming that no sulphur is lost from the green liquor or absorbed intoit in the quench. Further, the smelt is approximated to consist of onlyNa₂S and Na₂CO₃. K and Cl content is very low (cf. Table II) buthydroxide content at 1000° C. may be significant.

When the smelt composition obtained from the experiment is used topredict a melting point in the Na₂S—Na₂CO₃ phase diagram, a meltingpoint of approximately 850° C. is predicted compared to the 825° C.predicted for typical Kraft black liquor gasification smelt. Therelatively low melting point is a very important and encouragingconclusion, since it indicates that the risk for operating problems dueto smelt solidification may not be greater for sulphite thick liquorthan for Kraft black liquor. No signs of problems associated withdeposits caused by high smelt melting point were observed as assessedfrom temperature measurements, pressure drop between reactor and quenchand visual inspection of reactor after test termination.

Energy Efficiency

The energy efficiency can be measured by the cold gas efficiency (CGE),which is defined as the energy in cold synthesis gas divided by theenergy in the sulphite thick liquor. This measure shows how much of thechemical energy in the liquor that is transferred to the synthesis gasand is also an indication of potential biofuel yield. Higher heatingvalues (HHV) are used for the calculation in this paper.

Table V shows CGE values with and without adjustment of synthesis gasflow measurements. The adjustment is made in the synthesis gas flowmeasurement to close the mass for balance for C. The deviation, which is−6% based on the actual reading is probably explained by synthesis gasmeasurement uncertainty. It is known from experience that the gas flowmetering device can give too low readings due to clogging of pressuresensors. This is supported by an observed continuous decrease inmeasured gas flow in some of the operating points, although operatingconditions were kept constant. The gas flow reading used for balances istaken at the end of each operating period and is thus probably too low.An alternative mass balance with an adjustment of the synthesis gas flowto close the C balance has been calculated, which is referred to asalternative 2 when energy efficiency is discussed below. Note thatreactor temperature (and thus O₂/liquor ratio) is important for CGE andis included in the table for this reason.

TABLE V Gasification energy efficiency as cold gas efficiency MeasuredCGE HHV CGE HHV Operating RX temp gas flow Alt 1 Alt 2^(A) point C.Nm3/h % % 1 1010-1070 410 53.3% 54.5% 2 1010-1070 560 61.2% 60.7% 3 980-1010 550 61.0% 63.0% 4  980-1010 539 59.9% 61.7% 5 1010-1070 57857.2% 59.9% 6 1010-1070 563 56.6% 59.5% 7 1010-1070 552 55.4% 59.2% 8 980-1010 544 56.8% 65.5% ^(A)Alternative 2 is after adjustment ofsynthesis gas flow measurement to close C balance, see text.

Due to pilot scale effects, the CGE values in Table V are notrepresentative for what may be expected in a commercial scale gasifier.In order to estimate CGE for a full scale plant three adjustments may becompared to values measured in the pilot tests:

-   -   1. Heat losses may be decreased to a level that can be expected        in a full scale plant (≈500 kW at 500 tDS/d).    -   2. The energy required to heat N₂ in the reactor to the reactor        exit temperature may be subtracted since N₂ will not necessarily        be used in a commercial plant.    -   3. The results may be adjusted to account for the higher        efficiency reached at higher DS content (70% compared to 62%).

As in previous analyses, focus is on operating point 7 since this pointhas the longest duration and may be expected to represent steady-statebest. Adjustments according to item 1 and 2 for operating point 7increase CGE to 61% and 65% for alternative 1 and 2 respectively.

Adjustment to account for DS content may be less straightforward sinceit may require consideration to the lower O₂/BL ratio obtainable withhigher DS content. Simulations, based on a thermodynamic model of thegasification process, show that, at constant reactor temperature, theeffect on CGE by changing from 62% DS to 70% DS may be approximately 5%at the relevant operating point.

This leads to a commercial scale CGE estimate for conditions accordingto operating point 7 of approximately 66% and 70% for the twocalculation approaches respectively. It may be noted that the operatingpoint used has “high” reactor temperature and that acceptable greenliquor quality was observed also for lower temperature. Since CGEincreases with decreasing temperature, this indicates that even higherCGE may be obtainable but further experiments at higher reactor load isrequired to confirm this.

Discoveries:

-   -   Full carbon conversion may be reached at temperature and        residence time similar to what is used for sulphate liquors.        This is surprising since sulphite liquors normally have a        significantly lower reactivity in recovery boilers.    -   One explanation to the high carbon conversion is the efficient        usage of oxygen containing gas as atomizing medium, by        introducing the feedstock and oxygen to the same burner thus        achieving small thick liquor droplets and oxygen in good        contact, which leads to high flame temperatures and rapid        reaction rates.    -   Since oxygen or oxygen containing gas acts as atomizing medium        of the liquor feedstock no further addition of another atomizing        medium may be needed. Furthermore, no additional introduction of        oxygen or oxygen containing medium may be needed at other        locations of the reactor.    -   The rapid reaction rates also lead to the possibility of using a        lower global reactor temperature with maintained high carbon        conversion, which contributes to the high cold gas efficiency.    -   The fact that a specific atomization medium may not be required        also contributes to the high cold gas efficiency.    -   At least 98% or even as high as 100% sulphur reduction may be        achieved, i.e. significantly higher than what can be expected        with reference to experience from recovery boilers.    -   About 70% of the sulphur may be obtained in the produced syngas,        which is a surprisingly high number.    -   A slagging temperature (melting temperature) of the salt formed        in the reactor that may be kept lower than expected thus        improving the ability to make the salt melt flow out of the        gasifier.

The very good results leads to that a sulphite mill equipped withgasification of the sulphite liquor may:

-   -   Increase energy efficiency of the recovery process    -   Simplify the liquor cycle.    -   Drastically reduce or even avoid dead load of sulphate in the        liquor cycle.    -   Drastically reduce or even avoid losses of sodium sulphate from        the liquor cycle and decrease purchase of fresh sulphur and        sodium (as NaOH.)

As will be understood by those skilled in the present field of art,numerous changes and modifications may be made to the above describedand other embodiments of the present invention, without departing fromits scope as defined in the appending claims. For example, analternative process configuration and equipment design may be used toreach the same result if it is used for a slagging entrained-flowgasification process of sulphite thick liquor. It is also understoodthat the liquid phase produced in the gasification process as defined inclaim 1 should be constructed as also applying to a process comprisingsome minor amount of solid and/or condensed material, which may bepresent. The skilled man realizes that the concept is feasible also atatmospheric pressure and, furthermore, that the method also applies tobooster concept wherein the gasifier is operated in parallel with therecovery boiler.

1. Method for recovering chemicals and energy from sulphite thickliquor, said sulphite thick liquor being obtained when producing pulp bychemical delignification of fibrous raw material using a sulphitepulping process, said sulphite thick liquor comprising organic andinorganic compounds; the method comprising: processing said organic andinorganic compounds at a global temperature above 800° C. ad producingpartly at least one phase of a liquid material and partly at least onephase of a gaseous material; said processing is carried out bygasification of said sulphite thick liquor in a gasification reactor atsub-stoichiometric conditions and in the presence of an oxidizingmedium, wherein said reactor having an opening in its bottom in the formof a chute, which opens directly into a quench compartment, said chutehas a horizontal cross sectional diameter which is less than 40% of thelargest cross sectional diameter in a horizontal plane of said reactor.2. Method according to claim 1, wherein said chute has a horizontalcross sectional diameter which is preferably being 5-35% of the largestcross sectional diameter in a horizontal plane of said reactor. 3.Method according to claim 1, wherein said global temperature is at least900° C., preferably at least 950° C. and below 1300° C.
 4. Methodaccording to claim 1, wherein said gasification is an entrained flowgasification.
 5. Method according to claim 1, wherein the absolutepressure of the gasification process is about 1.5 to about 150 bar,preferably about 10 to about 80 bar, and most preferably from about 24to about 40 bar in the reaction zone.
 6. Method according to claim 1,further comprising introducing said oxidizing medium to an inlet of thereactor in vicinity of an inlet of the reactor for introduction ofsulphite thick liquor so that the oxidizing medium can be used foratomization of the sulphite thick liquor.
 7. Method according to claim1, wherein said oxidizing medium is oxygen gas or an oxygen containinggas.
 8. Method according to claim 1, wherein said sulphite thick liquorforming droplets when meeting said oxidizing medium, said dropletshaving an average droplet size below 300 μm.
 9. Method according toclaim 7, further comprising using steam as an atomization support mediumby introduction of steam to the reactor through a separate inlet or bymixing said steam with said oxidizing medium before said oxidizingmedium is introduced into the reactor through an inlet.
 10. Methodaccording to claim 1, wherein said liquid material is in the form a saltmelt, which is dissolved in a liquor thereby forming green liquor, saidgreen liquor being drawn off from said reactor and being furtherprocessed in order to convert sodium sulphide comprised in the greenliquor to sulphur dioxide and/or sulphite.
 11. Method according to claim10, wherein said sodium sulphide is converted first to hydrogen sulphideby contacting said sodium sulphide in countercurrent with carbon dioxideand sulphur dioxide, preferably in an absorption/desorption tower, andthen further to sulphur dioxide and/or sulphite.
 12. Method according toclaims 1, further comprising obtaining a green liquor having an amountof unburnt charcoal in said green liquor lower than 5%, preferably lowerthan 1% and more preferred lower than 0.2%, of the carbon in the sulfitethick liquor.
 13. Method according to claim 10, further comprisingobtaining a green liquor where sulphur found in said green liquor is toan extent of at least 90%, preferably at least 95% and more preferred atleast 98%, in reduced form as sulphide.
 14. Method according to claim 1,wherein said resulting gaseous material is a raw synthesis gascomprising hydrogen sulphide, carbon monoxide, hydrogen and carbondioxide.
 15. Method according to claim 14, wherein said hydrogensulphide and said carbon dioxide in said raw synthesis gas are beingremoved from said raw synthesis gas in an acid gas removal plant therebyforming a hydrogen sulfide rich stream and a stream comprising mainlycarbon dioxide from said acid gas removal plant.
 16. Method according toclaim 15, wherein said resulting hydrogen sulfide rich stream comprisesat least 25% hydrogen sulfide, preferably at least 35% hydrogen sulfideof its total stream content.
 17. Method according to claim 15, furthercomprising conveying the stream comprising mainly carbon dioxide fromthe acid gas removal plant to the recovery process.
 18. Method accordingto claim 15, wherein said hydrogen sulfide rich stream from said acidgas removal plant is being combusted directly with air or oxygen to givea combusted gas comprising sulphur dioxide, said sulphur dioxide beingabsorbed from said combusted gas in a gas scrubber.
 19. Method accordingto claim 1, wherein said sulphite thick liquor is a sodium basedsulphite liquor or a potassium based sulphite thick liquor.